This document describes the process of lymphopoiesis, or lymphocyte development. It discusses the development of lymphocytes from stem cells in primary lymphoid organs like the bone marrow and thymus, then migration to secondary lymphoid tissues where further differentiation occurs upon antigen exposure. Key cell types in lymphocyte development include lymphoblasts, prolymphocytes, small and large lymphocytes, and their defining characteristics. The roles of the microenvironment and cytokines in lymphopoiesis are also summarized.

1. Lymphopoiesis
LYMPHOBLAST
• Description:
Firstmorphologically recognizable cell oflymphocytic lineage, roundin shape. Under
normal conditionsit exists in secondarylymphoid folliculi.
• Size:
15-25µm.
• Cytoplasm:
Homogenous,basophilic,with the colora little bit more intensive at the margins; no
granules.
• Nucleus:
Roundorslightly oval; it has a fine dispersed chromatin whichis coarser thanin the
myeloblast. High nucleocytoplasmatic ratio (5:1 to 7:1).
• Nucleoli:
Only oneor twoblue colored nucleoli are present.
PROLYMPHOCYT E
• Description:
The secondmorphologically recognizable cell of the lymphocytic lineage, smaller than
lymphoblast but larger than mature lymphocyte.
Prolymphocytecansometimes be foundin the peripheral blood.
• Size:12-18µm.
• Cytoplasm:
Blue andrelatively more abundantthan in the lymphoblast, nogranules.
• Nucleus:
Coarserchromatinic structure,but,unlike the lymphocyte, chromatin is notclumped but
has a granular appearance.
• Nucleoli:
Poorlyvisible.
LYMPHOCYTE
• The cell that is normally foundinthe bonemarrow,lymphoid organsand in blood,
butin variouspercentages.
• Twotypes of lymphocytes canbe distinguished: small and large ones.
SMALLLYMPHOCYTE
• Description:
Small lymphocytes are the major partof lymphocytes in the blood.The cell has a large
roundishand regular nucleus.
• Size:
9µm.
• Cytoplasm:
Scantycytoplasm is often barely visible and quite often seen only at the nuclear
periphery. It may form a fine blue line aroundthe nucleusorhave the appearanceof a
pointed hat. The cytoplasm does notcontain granules.It is distinctly basophilic witha high
nucleocytoplasmic ratio (about9:1).
• Nucleus:
Eccentricallylocated, roundor oval, oftenwith an indentation in one place; chromatin is
heavily clumped,forming compactlumpyaccumulationswithout sharpdemarcations.
• Nucleoli:
Not visible.
• LARGE LYMPHOCYT E
• Description:
Largelymphocytes are the minor part oflymphocytes in the bloodstream. The cell has a
large roundishandregular nucleus.
• Size:
10-15µm.
• Cytoplasm:

2. Fairly abundantand sky-blue color;there may be upto 10azurophilic granules. They are
approximately 0.5µ in diameter.
• Nucleus:
Slightly eccentric located, roundoroval, sometimes slightly indented; chromatin is
arranged in compactblocks,separated by lighter zoneswithout sharpdemarcation, often
there are unclearly divided clumps.
• Nucleoli:
Not visible.
Lymphopoiesis
• Primary lymphoid organs(PLO)–BMand thymus
• Secondarylymphoid tissues (SLT)-LN,spleen, tonsils, peyer’s patches
• PLOsupplies the SLT withpartially differentiated lymphocytes
• Lymphopoiesis in PLOis continuousandantigen independent in SLT they become
immunocompetent.
• Immunocompetentcells develop along the defined pathways ofL differentiation
eg.BM – multipotential thymocytic progenitor(pro/pre-Tcell) – T cell
in BM – Bcell
NK cells fromtotipotent stem cells
• microenvironment – critical to its development andfunction.Effectsoflymphocyte
interaction with macrophages,fibroblasts, dendritic cells, endothelial cells.
• Soluble factors– cytokinesand interleukins (IL)
• (CAMs) celladhesion molecules – ligands orcoreceptorsfor surfaceproteinsand
facilitate adhesion to other cells
enables Lymphocytic cells to migrate, home andrecirculate…
Bone marrowlymphopoeisis
• Begins withtotipotent hematopoieticstemcells.
• Stem cells are foundin BM andcapable of self-replication. Once start to
differentiate becomes committed to a specific cell type
B cells
• Differentiation ofB pathwaysmay proceed in bonemarrow . Some precursorcell
may also migrate to peripheral organslike spleen
• Pro-B(progenitor)cell-earliest , CD19andTdT withoutIg
• Pre-B(precursor)cell–gene Ig rearrangement
dependent on IL-7– Heavy Chain Igexpression and upregulated by c-kit
initially they are large , rapidly dividing, with convolutednucleusandfollowed by a small ,
slowly dividing pre b cells. (infants,px withaplastic anemia orafter marrow transplantation,
LN after Agstimulation) – mononuclear,10-20um,largenucleus,homogenous chromatin,
cleft nucleus,deep blue scantycytoplasm w/ogranules and vacuoles
• Pre Bcell retain – HLA DRAg,CD19,some TdT
-rearrangement ofIg genes- m-heavy chains(HC) inpreB cells are evident
• Immature B cells – absence ofCD10 andTdT, 1st
appearance ofsurfaceIg(SIg),
intracytoplasmic HCof IgMtype,
{ CD20,CD22,CD40 appear} CD19isretained until plasma cell diff.
• Mature B cells- Light Chain(LC) aresynthesized joinedwith HCinserted in the
plasma membrane
LC–either lambda l, kappa k chains
but notboth
only mature B cells bind Ag– canbe activated andmultiply
Migrate to 2ndarylymphoid tissue (SLT)
where antigen-dependent phase ofdevelopment occurs
10%are precursors
90%are resting mature Bcells
Naturalkillercells
• Bone marrow-primary developmental site and also thymus
• Large granularlymphocytes
• + for CD2,CD11b,CD16,CD56antigensandnonadherent and nonphagocytotic

3. • CD16 andCD 56are the most aggressive
• Controlled by IL-2 cytokine
Thymic Lymphopoiesis
• Prothymocyte(pro-Tcell) originates from the CFU-Linthe marrow.
Agpresent like – c-kit, pan-T cell lineages CD2
andCD7, HLA-DRandcytoplasmicCD3(hallmark of T cell lineage)
Pro-Tcell-migrates to thymus,reacts with thymic environment (eg.Dendritic epithelial
cells , macrophages,intedigitating dendritic cells). Thymosinalso play onits differentiation
• Thymus
medulla ( central region),cortex(peripheral region ) enclosedwithin a capsule
cortex– large lymphocytes,rapidly dividing in absence ofantigens
medulla – immunocompetentcells
3 distinct intrathymic differentiation
1. subcapsular
2. common(cortical)
3. mature (medullary)
• Definitive marker for T cells are T cell antigen receptor (TCR)signallingcomplex
(TCR/CD3)
• TCRgene rearrangement
• CD4and CD8are acquiredin the cortexand2 subsets oflymphocytes eventually
develops
1. helper cells – CD4
2. suppressororcytotoxic cells – CD8
5% ofthymocytes leave the thymusand most migrate to SLT
• MHCmajor histocompatibility complex restrictions – important in Tcell
development
• T cells are programmed to recognizeAgonly whenit is linked to certain MHC
molecules on target cell surface(MHCrestrictions) eg. Helper T cell – MHCclass II
and cytotoxic cells are class I MHCrestricted
Secondarylymphoidtissue lymphopoiesis
• Germinal centersinside lymphoid follicles in the spleen ,LN , peyer’s patches , and
other mucosalassociated lymphoid tisssue (MALT)
• Newly formed lymphocytes are released by the secondarylymphoid tissue –
antibody response
Spleen
• Red pulp
• White pulp
• Marginalzone
• T cellsmove from marginalzone of the white pulp to PALS – migrate from
spleenfor only 5-6hours
• B cellsmove from marginalzone toPALS mixwith T cellsthento germinal
centersof W.pulpthen tovenous sinuses of the red pulp– 24hrs
Lymphnode
• Enclosedin acapsule withcontinuous endotheliallining.
• Afferent lymphaticvesselsthat penetratesthe capsule that carrieslymph
loadedwith Aginto the node to be filtered.Lymphentersthe subcapsularsinus
where it comesincontact withlymphocyte andmacrophages.The subcapsular
sinuses communicate withmedullarysinusesandemptyintoefferent
lymphatics
• Divided intocortexandmedulla
• Cortexcontainsprimaryandsecondaryfollicles(mantlezones)
• Medulla – cord like aggregatesoflymphocytesandplasmacells
2 routes of lymphocyte entryintoLN
1. afferent lymphatics
2. blood tru endothelialliningofpost capillaryvenules
• Thymusdependent zones- paracortex

4. • B celldependent zones-B cellsfrom germinalcentersmove tomedulla –
efferent lymphatics–viamedullarycordsin deepportion
• LymphoidfolliclesaccomplishT celldependent,antigeninducedBcell
proliferationanddifferentiation.
• Germinalcentersappeartobe the site of transformationof the B cellfromsmall
to large lymphocyte andleavesgerminalcenterasanimmunoblast towardsthe
medullary cordsto become plasmacytoidlymphocytesorplasmacells
Lymphatic aggregates
• Dense accumulationsoflymphocytesfoundinloose ct throughout GIT,RT,and
Urogenitaltractsassociatedwithmucosalsurfaces
• MALT
Lymphokinetics
• The process of lymphocyte multiplication, maturation,storage,andmigration
to tissuesincludingsitesof infectionor celldamage.
• Mitosis andMultiplication–radioisotope labeling
• BM fastest turnover,thymus– 2nd
Pre B cells– 0.6% in BM, 6%amonglymphocytes,3-4daystodevelop
T cells– supplied by BM precursors
• Lifespan
definingis difficult,most have short lifespan,influence byitsenvironment
high rate of productionand turnover
• Migration
adhering,traversing,and detachment fromendothelium
Traffickingorhoming– circulationandmigrationofcellstospecific tissues
Affectedby variousmolecules/mediatorslike –
(selectins,chemokines,chemoattractantreceptors,Igsuperfamilyeg.integrin
ligands,integrins)
HEV – high endothelialvenules
• Lymphocytesinteractwithendothelialcells, migrate intolymphoidornon
lymphoid
• Lymphocytessearchforantigens
• Once in the tissuesL move amongother cells( skin ,LN,spleen,lung, or
intestinalmucosa)
Function
• Major functionof lymphocytes
1. antigenrecognition
2. generationofappropriate immune response
B cells
function
• Diffrentiate intoplasmacellsthat secretesglycoproteinIgs(humoralimmunity)
• Producesvarietyof Igs
• Ig – removaland degradationof foreignbody (complement ,neutralization,)
• Produce cytokines–both B&T cells
• Mitogens– stimulatestransformationandmitosis
• Productionof plasmacellsandmemorycells
• Memorycells
T cell
function
• Effectorandregulatory function
• EffectorTcell –cellmediatedimmunitylike defense against intracellular
bacteriaorfungalinfections,cytolysisofvirusinfectedcells,allograft
rejections,GvHrxn, Tumorimmunity
• Subsetsof T cellsystem
1. Helper/InducerT lympocyte (TH) –CD4
2. DelayedhypersensitivityT L(DHTLor TD)
3. Cytotoxic TL(CTL) –CD4&CD8
4. Suppressor TL(TS)-CD8

5. Helper/ inducerT cell
• Induce other T celland B celltoproduce Antibody
• Release interleukins
• Physicalcontact –MHCclassII withspecialmarkerCD4
• IL2(CD25) – growth factorproliferationofactivatedT cellsdependent onIL1
releasedby macrophages
Delatedhypersensitivity
T lymphocytes
• Producedchemotactic lymphokineswhichconfine oractivate macrophages
(MIF) – migrationinhibitionfactor-IL4-membrane alterationscausingclumpingand
immobilizationofcells
(MAF)-macrophage activatingfactor-IF-g–antiviralactivity,inhibitionoftumor
growth, enhancement ofNKcellactivityandantibodydependent cell mediated
cytolysis(ADCC),alterationofcellmembranes,macrophage activation
• Recruit uncommittedlymphocytes
• Allergic reaction
Cytotoxic
t lymphocytes
• Both CD4 andCD8
• Induced toperform cytotoxic function
• Recognizedonly MHC classI
• Ability torecycle
Suppressor
T lymphocytes
• Regulate bothcellmediatedandhumoralimmunity
• CD8 as surface marker
• Inducer,effector,transducer
Large granularlymphocytes
• Large cellswithpale blue cytoplasm,lowNCratioandcytoplasmic azurophilic
granules
• 3 classes
1. naturalkillercells-NKcells
2. killercells-Kcells
3. lymphokine activatedkillercells-LAK
Naturalkillercells
• Function
1. recognitionandlysisof certaintumorcellsandvirus infectedcellswithout MHC
restriction
2. resistance tocertainbacterial,fungalandparasitic agents
3. immune regulation
4. regulationof hemopoiesis
5. naturalresistance toallogenic grafts
• Theyrecognizedtargetsnon-immunologically
• EnhancedbyIFN-g,IL2
• Target malignant cells,virusinfectedcells,fetalcells,subpopulationsof
thymuscells,BM cells,macrophages
• React withsyngeneic,allogeneic,xenogeneic
• Found in blood and spleen
Killercells
• Cytotoxic lymphocytes
• Antibodydependent cellmediatedcytolysis(ADCC)
• High affinityforIgG Fc receptor(FcR)
• Virusinfections,tumors,autoimmune disease
Lymphokine-Activated
Killercells
• ActivateddirectlybyIL2
• Phenotypicallydifferent fromNKcells

6. • Ability tomediate lysisoffreshtumor target cellsthat are resistant toNKcell
lysis
Cytokines
• Producedby specific andnon-specific helper/inducerL,cytolytic andsuppressor
TL,activatedT andB lymphocyte
• Proteinsthat functionto moderate effectorcellnumberandfunction.
• Types– interferons,tumornecrotic factor(TNF),Igsuperfamilymembers,
chemokines,hematopoietins(GM-CSF)
Monocytes
• Monocytesoriginate inthe bone marrow afterdifferentiationinto monoblasts
and promonocytes.Macrophagesare monocytesdistributedoutside the blood,
allover the body, especially inthe liver,spleen,lymphnodes,lungs, bone
marrow,connective tissue,andserous cavities.Monocytesphagocytize
bacteriaandlargerparticles;theycontaingreat quantitiesoflipase andcan
thus degrade bacteriawithlipidic capsule,suchastuberculosisandleprotic
bacteria.Monocyteshave adominant role in immunitybecause theypresent
the ingestedantigenon theirsurface so that it canbe recognizedby
lymphocytes.
• The maturationprocessand divisionin the bone marrowlast 16 to 26 hours.
Afterleavingthe bone marrow,monocytesremaininthe bloodstreamabout 7
days,then move into tissues,where theybecome macrophages.Theycan
proliferate toasmallextent,especiallyinthe place ofinflammatoryreaction.
Today it is indisputably clearthat the monocytesinthe blood and macrophages
in the tissues are the same cellspecies.Theyare classifiedasmononuclear
phagocytes.
Developmentalstagesofmonoblasts
• MONOBLAST
• Description:
The most immature precursorwithmorphologic featuresofmonocytic lineage and
the first morphologicallyrecognizablecellofthe monocytic lineage,roundoroval
shape.
Monoblast is instructure verysimilarto the myeloblast fromwhich it cannot be
distinguishedon morphologicalgrounds alone,evenby meansof the electronic
microscope.Theycanbe differentiatedusing cytochemicalmethods.
• Size:
20 µm.
• Cytoplasm:
Blue or greyishand no granules.
• Nucleus:
Large,round or slightlyindented,andthe chromatinis veryfine and indistinct.
• Nucleoli:
1-2 nucleoli.
• PROMONOCYTE
• Description
The second recognizable cellofthe monocytic lineage,roundor ovalshape.
• Size:14-20µm
• Cytoplasm:
Bright blue and maycontaineitherfewor manyfine azurophilic peroxidase negative
granules.
• Nucleus:
Large,indentedor notched,irregularin form,and shows some protuberancesin the
cytoplasm.
• Nucleoli:
Clearlyvisible.
• MONOCYTE
• Description:
The cellis round or oval shape. Because ofameboidmovementson the edge of the
cell1,2 or more pseudopodia canbe seen.

7. Monocytesenterthe blood and migrate tothe tissues to become macrophages.Unlike
neutrophils,monocytesinthe peripheralblood retainmitochondriaandribosomes,so
theyare able to synthesizeproteins.
Monocytescanbe found in the peripheralblood of healthyadults,inthe percentage of
1-9%, and in about 9% in children.
• Size:15-30µm
• Cytoplasm:
Relatively abundant dullgrey-blue;mayhave aground-glass appearance witha
numberof evenly distributedfine azurophilic granules.
• Nucleus:
Usuallyround or kidney-shapedbut maybe markedlyindentedoreven lobulated,
withtwo or more lobes. The chromatinisarrangedin strands with lighterspacesin
between,givingaloose skin-like appearance.
It is locatedsubcentrallyormore towards the peripheryof the cell.As arule,the
concave side of the nucleusfacesthe widest part of the cell.
• Nucleoli:
Not visible.
• MACROPHAGE
• Description:
Cellswithremarkable ability ofphagocytosis.
Phagocytesare dividedintotwo groups: polymorphonuclearphagocytesand
mononuclearphagocytes.The latterare macrophages,monocytes,promonocytes,and
theirprecursors in the bone marrow.
Monocytic-macrophagocytic systemhasasignificant functioninthe organism - it
eliminatesantigens,presentsantigentolymphocytes,andsecretesthe factors
stimulatinglymphocytestoreact more powerfully.
Macrophage is acellof various shapes,round or kidney-shapednucleus,withnumerous
cytoplasmaticinclusionsandirregularsurface withalot of folds and protuberances.
• Size:10-25µm
• Cytoplasm:
Numerouscytoplasmaticinclusions.Macrophagescandifferinthe amount,
structure and locationofcytoplasmatic enzymes.
• Nucleus:
Kidney-shapedor oval.
• Nucleoli:
Absent.
###Monocytosis
• Monocytosisis the state ofexcessmonocytesinthe peripheralblood. It maybe
indicative ofvariousdisease states.Examplesofprocessesthat canincrease a
monocyte count include:
• 1.chronic inflammation
• 2.stress response
• 3.hyperadrenocorticism
• 4.immune-mediateddisease
• 5.pyogranulomatousdisease
• 6.necrosis
• 7.red cellregeneration
• 8.ViralFever
• 9.sarcoidosis
• >Ahigh count of CD14+CD16++monocytesisfound in severe infection
(sepsis) and avery low count of these cellsis found aftertherapywithimmuno-
suppressive glucocorticoids[6]
###Monocytopenia
• Monocytopenia isa formof leukopenia associatedwithadeficiencyof
monocytes.